A series of N-and C-protected, monodispersed homo-oligopeptides (to the pentamer level) from the cycloaliphatic C Y -dialkylated glycine 1-aminocyclononane-1-carboxylic acid (Ac W c) and two Ala/Ac W c tripeptides have been synthesized by solution methods and fully characterized. The conformational preferences of all the model peptides were determined in deuterochloroform solution by FT-IR absorption and I H-NMR. The molecular structures of the amino acid derivatives mClAc-Ac W c-OH and Z-Ac W c-OtBu, the dipeptide pBrBz-(Ac W c) P -OtBu, the tetrapeptide Z-(Ac 9 c) 4 -OtBu, and the pentapeptide Z-( Ac 9 c) 5 -OtBu were determined in the crystal state by X-ray diffraction. Based on this information, the average geometry and the preferred conformation for the cyclononyl moiety of the Ac 9 c residue have been assessed. The backbone conformational data are strongly in favour of the conclusion that the Ac 9 c residue is a strong b-turn and helix former. A comparison with the structural propensity of a-aminoisobutyric acid, the prototype of C Ydialkylated glycines, and the other extensively investigated members of the family of 1-aminocycloalkane-1carboxylic acids (Ac n c, with n 3±8) is made and the implications for the use of the Ac W c residue in conformationally constrained analogues of bioactive peptides are briefly examined.